In order to be able to interpret arterial blood gases and acid base balance effectively one needs to have a basic understanding of some of the physiology underlying the principles involved in an analysis.
These principles may appear complicated at first but it is very much worth persevering when trying to understand them as then one will be rewarded with a greater understanding of the blood gas you are presented with an some of the physiological processes underlying that gas.
Rather than calling this section arterial blood gas analysis, it should more correctly be called acid base balance as it is this which underpins a lot of the physiology we are about to discuss.
Acid base balance is about the body’s attempts to maintain its pH between very strict parameters. It is within these parameters that the bodies functions perform best. Outside of these parameters there can be issues such as problems with the proteins within the body, which are crucial to its function.
So what is pH? PH is a measure of the acidity or alkalinity of a liquid. PH is indirectly proportional to the number of hydrogen ions within that liquid. So, as the hydrogen ions rise so the pH will fall. This relationship is described by the formula below:
pH = -log [H+].
Whilst one can use both the hydrogen ions and the pH to describe a patient’s acid-base status it is more common to use the pH level only.
So what is the patients normal pH meant to be? Well the normal pH should fall between the range of 7.35 to 7.45. If the pH is lower than 7.35 the patient is said to have an acidaemia. Conversely if the pH is higher than 7.45 the patient is said to have an alkaleamia. However knowing this will only tell you that the pH is either less than 7.35 or greater than 7.45, it will not explain which process is causing this problem.
Maintaining normal pH
In order to try to understand which process is causing the problem one first has to understand how the body deals with the hydrogen lines within the blood. Unfortunately we have to once again use a bit of chemistry to understand this process;
H20 + CO2 ↔ H2CO3 ↔ H+ HCO3
You will notice from the double ended arrows in this formula that this is a process that can move both ways.
At the right end of the formula of the hydrogen ions which can combine with the bodies natural buffer, bicarbonate. This then forms carbonic acid, which can then be moved around the body without unduly affecting the body is pH. It can be moved around to the lungs, where it dissociates into water and carbon dioxide, the carbon dioxide (which can be classed as an acid) can then be breathed out.
Or it can be moved to the kidneys where again it dissociates into bicarbonate and hydrogen ions and can be excreted into the urine.
So you can see from this formula that the body deals with excess hydrogen ions in a couple of ways. Essentially it can either breathe them out by the lungs or it can excrete them via the kidneys as urine.
Lungs and Kidneys
As a consequence the acid-base balance has a respiratory component, the lungs, and a metabolic component, the kidneys.
One is able to increase one’s respiratory rate quite quickly in order to respond to an acid-base imbalance. So if the hydrogen ions begin to rise the patient’s respiratory rate can increase, which will excrete the carbon dioxide. So this is quite a fast response.
The kidneys dictate the levels of bicarbonate within the body, but compared to the lungs can only change the levels of bicarbonate quite slowly. So if, for example, the carbon dioxide within the body begins to rise because of a chronic lung condition the kidneys will take some time to compensate for this by increasing the levels of bicarbonate. So this is quite a slow response.
The last two paragraphs are important principles to remember when discussing acid-base balance mainly because one can tell from an arterial blood gas whether the respiratory condition the patient is suffering from is an acute or chronic condition. Through some of the examples given hopefully this will become clearer.